


you are a fever

by nilchance



Category: Undertale (Video Game)
Genre: Dubious Consent, Hatefriends with Benefits, M/M, Mating Cycles/In Heat, Underfell Sans (Undertale), kustard - Freeform
Language: English
Status: Completed
Published: 2018-09-17
Updated: 2018-09-17
Packaged: 2019-07-13 16:06:33
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,140
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16021343
Author URL: https://archiveofourown.org/users/nilchance/pseuds/nilchance
Summary: Sans goes into heat. Red is as helpful as he ever is.





	you are a fever

**Author's Note:**

> Based on a prompt from somebody on tumblr who I think preferred to be anon (?) for a fic where Sans goes into heat and hides it only for Red to find out. This is a standalone, not set in any particular universe. Dubcon just for the nature of heatfic. One day I will write a kustard that isn't basically Homestuck-style blackrom but today is not that day.

There's a knock on the door.

Sans's mind is a slow morass of heat and need and ache. He's sweated through his clothes until he finally stripped to the bone. Day two and it's getting harder for anything to penetrate. Trying to sound vaguely alert, he slurs, "m'fine, paps. still okay. just lemme sleep, huh?"

His voice is shredded from not talking, aside from muffling curses into the hoodie he shoved into his mouth as he jerked off with more optimism than realistic expectations, but he coughs for some vague authenticity. Yep. Only flu victims here. Absolutely 100% normal.

"it ain't paps," says an aggravatingly familiar voice. "you gonna unlock this or am i shortcutting in?"

Oh. Red. Fuck that guy.

"go away," Sans says.

Which is a little ruder than he likes to be where Papyrus could conceivably hear him. Papyrus wants everybody to get along. He likes Red. But then Papyrus tries to like everybody.

It's not like Sans hates Red. They just rub each other the wrong way. Too much alike not to. That whole 'I don't like you but we're stuck with each other' thing seems to be mutual. So they restrain their sniping to jokes that are just a little too sharp and stay out of each other's way, aside from when they need to gang up on one of the others to get them to take care of themselves. Older brother tag team champions.

"okee dokee," Red says.

Then there's a pop of air pressure shifting and Red's standing in his bedroom like the bottom of an emo teenager's closet gained sentience and decided to haunt him.

With limbs that are operating on a five second delay, Sans grabs clumsily for his hoodie and shoves it between his legs to cover the junk he's had manifested for the last day and a half. He snaps, "naked!"

Red gestures at his own pelvis. "seen it, genius. looks just like mine. flu, huh?"

"get out," Sans says. It comes out weaker than he wants it to. He hasn't really slept since this started and the sight of another person, even Red, hits him right in the hindbrain. He _wants_.

"you dumb motherfucker." Red comes over and sits himself down on the edge of Sans's mattress, casual as if it's his spot at Grillby's despite the fact that the air is heavy with the smell of sex and there are wet spots on the mattress barely covered by Sans's hoodie. "didja really think you were being slick?"

Red smells amazing.

"really not a good idea for you to be here," Sans says. He should push himself away from Red. A better version of him in some universe somewhere probably does. He stays put and tries to breathe through his mouth, but that just means he tastes Red, smoke and sweat and bone.

"laid up for two days," Red scoffs, as if Sans isn't a sweaty, shaking knot of bones next to him. "like that wouldn't get everybody up your ass even if you were just a normal kind of sick. we ain't so good at boundaries, y'know?"

"did you want something?" Sans says through his teeth.

"you taleverse monsters usually don't gotta deal with it," Red says. "guess you caught wind of it when slim went into heat last week. happens like that sometimes, cycles syncing up and shit. sucks, don't it?"

Sans glares at him. He's burned through most of his already limited stamina so some exhausted glaring is the best he can do. "did you come to gloat?"

"nah. that's just a bonus." Red grins down at him, all Cheshire cat. "your bro's not here. he's gonna crash with stretch and blue for a day or so. you wanna fuck?"

The bluntness is unsurprising, coming from Red, but it jolts Sans. His mind is suddenly full of lurid technicolor pictures of how very much he wants to, almost crowding out how suspiciously generous it is of Red to offer. Generous isn't the first word that comes to mind when Sans thinks of Red.

(That's bullshit. He's seen Red with Frisk, buying them food, shrugging off his jacket and slinging it around their skinny shoulders when he thinks they're cold. He's seen Red with Papyrus, Slim and Stretch. That streak of protectiveness he has. That unexpected gentleness. He doesn't want to see it but he does.)

"don't get me wrong here," Red says. "maybe i just got a kink for dudes in heat. you're still an asshole. this doesn't change a goddamn thing."

Also bullshit. Red offering to help him out has already changed things. But Sans is fried. Slinking off like an old cat into the woods to either get better or die in relative privacy was a bad call. He's smart enough to know that.

Red sighs impatiently. "look, you stubborn dick, i ain’t thrilled about it either but you gotta pick someb--"

Sans grabs Red by the shoulder, half pulling him down and half lurching up to kiss him. It's clumsy as hell but Red opens for him, pushing Sans back down to the mattress. It's so good to have Red's hands on him, Red's tongue brushing against his, settling his restless jitters.

When Red pulls back, Sans has to fight not to drag him back down. He grips the mattress cover instead. Red glances at his hands, then grins crookedly at him, his eyes hooded. "okay, maybe this could be fun."

There hasn't been a single fun part of this, from the first second that Sans started to feel restless and too hot to now, when he's struggling not to be a complete handsy asshole as Red peels off his jacket, then his shirt.

As Red shifts to pull off his shorts, revealing inch by inch of bare scarred bones (Sans's fingers are digging into the mattress hard enough to hurt) he says casually, as if this happens every day, "you wanna fuck me?"

"thought we established that," Sans says. He can't pull his attention off Red's pelvis (the only part of him that's completely unscarred), the haze of magic there. It's the same shade of red as oxygenated blood, vivid and unambiguously lewd. His mouth waters. His magic is so primed for this it's painful, like he hasn't come a dozen times since yesterday. He might be trembling.

"do you want to put your dick in my cunt?" Red asks, overenunciating each word like Sans is an idiot. "or do you want me to put mine in yours?"

Sans's face floods with heat. Red laughs but before he can say anything about Taleverse monsters being hilariously prudish, Sans says, "that first one right now. the other one later."

"planning ahead," Red says, a glint in his eyes. "nice. now lemme see what i'm dealing with here."

Without preamble, he pulls the hoodie away from Sans's pelvis. His gaze is frank, assessing. Despite what Sans kind of expects, Red staring at his junk like he's considering whether to buy a used car doesn't make him any less hard. A bead of precome wells up and drips from the head of his cock to paint the bones of his pelvis. Then Red's eyes drag down to the space below Sans's dick, where his overenthusiastic, overheated magic formed a cunt too. Sans has no doubt the inside of his femurs are a fucking mess of blue. Red's feral grin sharpens.

"would you just fucking--" Sans snaps.

In one smooth move, Red straddles Sans's pelvis. Automatically, Sans grabs him by the hips. Red's iliac crests fill his hands, warm to the touch, and Sans thinks he's digging his fingers in but he can't loosen his grip. The bones under his hands heat more as Red's magic solidifies into the shape of a pussy. Red's position spreads the lips open a little, showing a glint of slickness there. Maybe he wasn't kidding about that kink thing. The scent rises between them and Sans shudders hard, feeling the pulse of fresh wetness sliding from his own pussy like sympathy. He's definitely digging bruises into Red's hips now but he manages somehow not to roll him over and just drive blindly into him until he can find relief in that wet heat. 

Red's cool fingers curl around his cock, guiding him in the right direction. Even as hard and desperate for a touch as it's been for hours, since he gave up on trying to relieve the heat himself, as good as it feels, his exhausted body fighting the same climax it's relentlessly demanding. But he arches helplessly into Red's fingers with an, "oh fuck."

Red grinds his hips down once into Sans, sliding his dick between the lips of Red's pussy, dipping just in, giving him a taste but not letting him have it. When Sans opens his mouth to ream him out because _what the absolute fuck_ , Red says matter-of-factly, "i'm gonna ruin you."

Red pushes down on him, taking him in one easy roll of his hips. It's a good goddamn thing that the house is empty because Sans makes the loudest noise he's ever made in bed, and he makes it for Red. Red the asshole, Red the murderer, Red who is the most aggravating person Sans has ever met, is hot and tight and perfect inside.

Red groans deep in his chest, his head falling back, bracing himself on Sans's ribs. It's a filthy sounds, ripped straight out of some porno somewhere. When Red rocks his hips, grinding on him, Sans moans helplessly and Red grins. At least his voice is rough and a little shaky on the edges, not that anybody but them could tell. "y’know, i wondered what you'd be like in the sack. thought you'd be a wild ride. meek on the streets, freak in the sheets. but you're so. fucking. _polite_."

Red punctuates the last three words by tightening around him hard. Sans curses, turning his face away as sharply as if Red slapped him. There's a sweet tension building in him like his body's forgotten about that whole tired thing. Red is about seven shots of espresso in a row, bitter and hot and impossible to shrug off. He tightens his grip, trying to move Red on him, to steer him into going faster.

Red laughs and goes back to grinding on him slow, like they have all the time in the world. "two days of heat. you gotta be dying but you're letting me fuck with you, ain't you? you're choking yourself with your own damn leash."

"maybe i'm just trying not to be a fucking dick," Sans bites out. His pulse is racing with sex and sheer fucking aggravation.

Red shifts on top of him, bringing their bodies together so he can say against Sans's mouth, "you know you wanna roll me over and fuck me through the mattress. so go on, sansy. do it."

Disbelieving, Sans demands, "you really wanna play mind games right now?"

“sex is the best time to play mind games.” Red tightens around him again, deliberate, digging his fingers into Sans's ribs. The pain and the pleasure are all just nerves firing and Sans makes an involuntary noise, hips jerking up into Red. Red purrs satisfaction but _doesn't fucking move_. When Sans breathes in to curse at him, Red bites his jaw, another little jolt of sensation, and says, half-laughing, "do it, you fucking coward."

Something in Sans snaps.

In one move, momentum and gravity magic and Red's complete lack of resistance, Sans reverses their positions. His hands on Red's shoulders pin him down hard and Red only laughs and hooks his legs around Sans's hips. His laughter hitches into a grunt as Sans pushes back into him. Sans hesitates, his body screaming at him for it, and Red digs his fingers hard into Sans's back. "c'mon, i ain't gonna break. fuck me like you mean it."

Which is honestly such a terrible line but Sans only really cares about the part where Red consents. He braces himself, pressing his face against the place where Red's neck meets his shoulder, breathing in the scent of him as he drives into Red hard. Red groans, rich and deep, and the dark part of Sans that is behind the steering wheel right now is satisfied.

Well. Almost satisfied.

He shoves a hand down between them and finds Red's clit. His hands are graceless, no finesse, but Red jerks under him, hips tilting pleadingly up into Sans. Maliciously, Sans slows down his thrusts. He likes the pleasured noises Red makes to an unfortunate extent. Unsteadily, Red says, "got myself nice and worked up for you before i came in here."

The mental image is a flashbulb in Sans's brain, bright and clear. Red going into their bathroom as soon as Papyrus was gone, sliding a hand into his shorts to rub at his pubic symphysis. Would he do it fast, business-like, or would he take his time and enjoy it? Did he just do it in the hallway outside the door? If Sans took Red's fingers in his mouth, would he taste him?

It's too easy to think of Red this way. Too natural, the unreality of the last few minutes replaced by the fact that he has Red underneath him, breathing faster. Sans starts fucking him fast, that sweet tight heat around him, and Red grabs at the back of his neck, panting. Sans can hear how wet he is. Red’s voice echoes off the walls, short little moans punched out of him instead of all his bullshit words. It’s a definite improvement. When Sans bites his collarbone, Red shouts, shamelessly loud. Sans's body sings, all his exhaustion forgotten in the flood of endorphins.

"fuck, 'm gonna come," Red says, the words blurring together.

Sans doesn't need the announcement, he can feel Red's cunt starting to clench, but he says against Red's throat, practically growling, "good."

Red laughs, a little wild, and uses his grip on Sans's spine to drag him over to his throat. A shudder runs down Sans's back, throwing off his rhythm, and he closes his teeth on Red's spine. He doesn't try to gentle it. He doesn't try to be reasonable. He bites hard enough to leave a mark.

"shit," Red hisses, pressing Sans's mouth to him. "oh, you bastard, you fucking--"

The rest of the words cut off in a moan. Red tightens down around him, almost painful, pressure and heat and friction all at once combining to wrench the orgasm out of Sans by force. It's not the resentful there-and-gone climaxes he's managed since this started, unsatisfying and empty. It shakes him down to his atoms, rising and rising until he thinks it might break him apart. The noise he makes is small and hurt.

It recedes. Finally, it recedes, after he spills who fucking knows how much jizz inside Red. He leans his brow against Red's shoulder, breathing hard. At least Red sounds just as winded. The hand on Sans's neck stays put for a while, maybe a minute of what Sans refuses to call the afterglow, until Red finally shoves at his shoulder. "all right, get the hell off."

That feral part of Sans really doesn't want to. He rolls off anyway, flopping onto his back beside Red on the mattress, but he keeps one eye on Red. The heat is quiet but he can still feel it smoldering, ready to flare up. Expecting it to be fixed with one round of fucking is probably a little overoptimistic.

Red touches his throat. There's definitely a mark, a pink bruise already forming on Red's spine. Sans's soul feels hotter at the sight of it.

"aww," Red says, fingering the mark. "i knew you had it in you."

"pretty sure you're the one who had it in you," Sans says mildly.

Red snorts. Taking his hand off his throat, he reaches down between his legs. Sans's pulse jumps as Red slides his fingers along his slit, gathering up the mess of various fluids there. Red winks at him and theatrically licks his fingers clean. "i sure did."

"why are you like this?" Sans asks, half-disgusted and half turned on. That's starting to become a common theme with Red.

"hey, i said i'd help you out. you didn't think i wasn't getting something out of it, did you?" Red rolls onto his side to grin at Sans like the weirdest Playboy model ever. "i mean, it was 50/50 whether you’d be lousy in the sack. maybe 60/40 and not in your favor. but if you couldn’t get me off, we’d both know it and i’d have ammo forever.”

“thanks so much,” Sans says. “kinda worried you thought about it that hard.”

“i got insomnia,” Red says. “thinking about your sex life knocks me right out. that heat building up again?"

As if Red summoned it into being through sheer smarm, Sans realizes that he's starting to feel a little restless. Unsatisfied. The worst part is that he's not dreading the thought of this dragging out for another round. Maybe two. Maybe four.

"yeah.” Sans shoves at Red's hip. "roll on your back."

"you don't even wanna change positions?" Red asks, like he's not already doing what Sans asks. His legs fall open. Sans can see the blue trickling out of him and his soul clenches in a hot wave. "borrrrring. 'course, i guess i shouldn't be surprised--"

Ignoring him, Sans moves to lay between Red's femurs, repositions one of Red's legs over his shoulders, and licks a broad stripe up Red's slit. Red says, "holy _shit_ , dude," caught gratifyingly off guard, and Sans grins against him. When he drags his tongue over Red's clit, Red grabs the back of his head and grinds up against his face like the rude asshole he is.

"you kinky bastard," Red breathes. Sans doesn't expect Red to tremble when he slips his tongue inside him but Red does, fingers scraping across the back of Sans's skull. "you've been holding out on me."

Sans pulls back to replace his tongue with two fingers. Red takes him easily, gasping, his heel digging into Sans's back. The heat and the taste and the sweet noises Red makes as Sans grinds ruthlessly against his g-spot go straight to Sans's head. He says, "i'd have done it before if i knew it'd shut you up."

Red grins, all shark teeth and bright eyes. "it won't. but hey, feel free to keep trying."

"replication of results," Sans says. He's grinning back, a little manic.

"for science," Red says. His fingers feel like they're digging bruises on the back of Sans's head. The scent of him is like a drug, making Sans's pulse go too fast. He feels alive.

"finally," Sans says. "something we can agree on."


End file.
